Electronic motor protection apparatus

ABSTRACT

A electronic motor protection system is shown having a power supply (12) usable with multivoltage supplies and providing protection from over-temperature conditions caused by various fault conditions. The system also has optional features of a time delay (20) for re-energization and for low voltage protection (22). The system is mounted on a circuit board and encased in potting material which also locks two housing parts together (30, 34). A miswiring feature is also provided to prevent connecting line voltage to the control circuit or sensor terminals.

This is a continuation-in-part of Ser. No. 08/260,627 filed Jun. 16,1994 now abandoned in favor of continuation application Ser. No.08/444,446; Ser. No. 08/260,627 being a continuation application of Ser.No. 07/943,254 filed Sep. 10, 1992, now abandoned.

This invention relates generally to protection apparatus for electricmotors and more specifically to electronic protection systemsparticularly suitable for compressor motors.

Systems for protecting dynamoelectric machines from over-temperatureconditions comprising one or more thermal sensors, such as positivetemperature coefficient (PTC) of resistivity sensors, disposed in heattransfer relationship with the windings of the motor are well known inthe prior art. It is also well known to add other protective functionsto such systems which will protect various apparatus from the occurrenceof additional conditions in an economical and efficient manner. Forexample, once the supply of power to a load has been interrupted, eitherthrough the sensing of an over-temperature condition by the PTC sensorsand concomitant tripping of the protection circuit, the opening of acontactor or for any other reason, it is undesirable to permitreapplication of power to certain loads, such as air conditionercompressors, until after a certain period of time has elapsed therebypermitting the load to return to a condition suitable forre-energization. In the case of compressors a short time delay allowsequalization of pressure in the compressor so that less starting torqueis required.

A system which accomplishes the above functions in an effective manneras well as providing additional protective features such as oil pressureprotection to ensure that the system is not operated when oil pressurefalls below a safe value for more than a selected time interval and alow voltage feature to avoid the possibility of causing contactchattering or having one phase of a multiphase motor drop out whichwould cause overworking of the remaining phases is shown and describedin U.S. Pat. No. 4,281,358 assigned to the assignee of the presentinvention.

In the system described in the referenced patent the protection systemhas a power supply including a conventional transformer to convertoutside supply voltage to a selected lower voltage suitable foroperation of the control circuit. In that patent the control circuitutilizes a 24 volt supply so that separate transformer means, or atleast a transformer with selected taps has to be utilized with differentline voltage supplies. This results in having to provide differentmodels of the protection system for different line voltage applications.

It is an object of the invention to provide an electronic motorprotector providing multifunction protection which can be used with aplurality of different line voltages. Another object is the provision ofan inexpensive yet reliable motor protection system. Still anotherobject is the provision of such a system which is compact in size andlight in weight.

Briefly, in accordance with the invention, a solid state power supplycomprising an integrated circuit having a first switching pre-regulatorsection and a second voltage regulator section is used to provide aselected DC voltage. The switching pre-regulator conducts for a portionof the positive half of the AC line cycle which is used to charge acapacitor which in turn is coupled to the voltage regulator to provide afirst level of DC voltage. An inhibit capacitor is coupled to thepre-regulator and is adapted to turn off the pre-regulator on theoccurrence of transients exceeding a selected dv/dt rating. According tothe invention the first level of DC voltage is used to directly drive arelay to control energization of the motor contactor. The first level ofDC voltage is in turn stepped down by a limiting resistor and a zenerdiode to provide a second level of DC voltage for energizing the controlcircuitry.

The control circuitry comprises a plurality of cascaded operationalamplifiers in the form of comparator sections providing a single channelincluding an over-temperature protection section in which one or morePTC thermal sensors are coupled to the inverting input of a firstcomparator. The output of the first comparator is coupled to a buffercomparator which in turn is coupled to the base of a transistor seriallyconnected to the relay coil. The output of the first comparator is alsooptionally connected to a time delay comparator whose output is coupledback to the inverting input of the first comparator. The output of thefirst comparator is also optionally coupled to the output of a lowvoltage comparator.

According to a feature of the invention the protection system is mountedon a circuit board received in the bottom half of a housing which isencased with potting material to provide vibration insensitivity,environment protection and to meet agency standard spacing requirements.The top half of the housing has a plurality of ribbed depending skirtsadapted to extend into the potting material to lock the housing partstogether. According to another feature of the invention the input bladeline terminals of the system are configured smaller than the sensor oroutput terminals to obviate any possibility of miswiring by putting linevoltage across the control circuit or sensor circuits.

According to an alternate embodiment, nuisance tripping of thepre-regulator section of the integrated circuit power supply caused bycertain conditions common in industrial environments is avoided byeliminating the inhibit capacitor which had been tied to ground andinstead using a diode and capacitor combination tied to the high side ofline voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages and details of the novel and improvedprotection apparatus of the invention appear in the following detaileddescription of the preferred embodiment, the detailed descriptionreferring to the drawings in which:

FIG. 1 is a schematic circuit diagram of the protection apparatus madein accordance with the invention;

FIG. 2 is a voltage vs time graph showing line voltage and voltage atthe IC pin of the switching pre-regulator;

FIG. 3 is a blown apart perspective of the upper and lower housings ofthe protection apparatus made in accordance with the invention;

FIG. 4 is a cross sectional view taken on line 4--4 of FIG. 3;

FIG. 5 is a top view of an assembled protection apparatus shown in FIG.3.

FIG. 6 is a schematic circuit diagram of a modified power supply sectionof the protection apparatus made in accordance with a modifiedembodiment of the invention; and

FIG. 7 is a functional diagram of the power supply integrated circuitused in FIGS. 1 and 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, protection apparatus 10 made in accordancewith the invention comprises a power supply section 12, over-temperatureprotection section 14, output circuit 18, time delay section 20 and lowvoltage cut-out section 22. Section 14 includes operational amplifiercomparators 1 and 4 and sections 20 and 22 include operational amplifiercomparators 2 and 3 respectively, each comparator having an NPN outputBJT transistor (open collector). During steady state operatingconditions the output transistor of comparator 1 is off, the outputtransistor of comparators 4 and 2 are on, and the output transistor ofcomparator 3 is off. As will be explained in detail below, when anover-temperature condition occurs it will cause the output transistor ofcomparator 1 to turn on and the output transistor of comparators 4 and 2to turn off (comparator 3 still being off). When low voltage conditionsoccur the output transistor of comparators 1, 4 and 2 are off and theoutput transistor of comparator 3 turns on.

With regard to specific sections of circuit 10, power supply section 12comprises a fuse F1 connected to the AC high, terminal T2 as a safetyfeature so that in the event of a short circuit the apparatus will failwith an open circuit. A surge suppressor varistor MOV1 is coupled acrossthe AC high terminal T2 and AC return terminal T1 to clamp highfrequency and high energy transients such as lightning surges. ResistorsR1, R2 and capacitor C1 connected across the AC lines serve as a lowpass filter to filter high frequency noise and line spikes.Additionally, resistors R1, R2 serve as current limiting devices for theSCR based pre-regulator to be discussed below.

Circuit section 10 includes a single chip power supply, integratedcircuit 24. IC 24 has eight pins only seven of which are utilized, pin 7being unconnected. As seen in FIG. 7, IC 24 comprises two sections, aswitching pre-regulator 24a and a linear post or voltage-regulator 24b.Pin 8 serves as an input pin and is connected through current limitingresistors R1, R2 to T2, an AC high line and internally to internalswitch 24c which connects pin 8 with pin 2. Pin 2 is also connected tothe input of voltage regulator section 24b. An external energy storagecapacitor C2 has one side connected to pin 2 and its opposite sideconnected to the AC return T1. Pin 1, connected internally to pin 3, isalso connected to AC return T1. When the input voltage goes positive theinternal switch 24c comprising a diode DA1 connected to the mainelectrodes of an SCR SA1 allows current to flow charging capacitor C2.Switch 24c also comprises an SCR SA2 whose main electrodes are connectedto the gate of SCR SA1 and to pin 1. SCR SA2 has a second anodeconnected to an internal zener diode/rectifier diode stack, voltagereference circuit 24d, which is used to be compared with the outputvoltage at pin 6 during the charging of capacitor C2. When the voltageon capacitor C2 reaches a level of the output voltage plus the voltageacross reference circuit 24d the switch 24c is turned off and chargingof capacitor C2 stops.

Voltage regulator section 24b takes current from capacitor C2 to providean output voltage on pin 6 which is set by feedback to sense pin 5 whichis connected internally in IC 24 to amplifier stage 24e and in turncoupled to output pin 6. The output will rise to a voltage levelnecessary to keep sense pin 5 at 5 volts and may be adjusted above the 5volt reference by placing a selected resistance such as resistor R4shown in FIG. 1, or zener diode, between pins 5 and 6 (an additional 1volt is provided for each one thousand ohms so that resistor R4 of 19Kohms provides a 24 Vdc output on line 26 used in the preferredembodiments for relay coil K1 and associated contacts M1, M2. The sameoutput would be accomplished via a 19 volt zener diode).

Pin 4 is coupled internally in IC 24 to voltage reference circuit 24dand when shorted to T1 will turn switch 24c off. More specifically, pin4 is connected to a second anode of SCR SA2 providing a safety featurekeeping switch 24c off during large dv/dt transients by means ofconnecting an inhibit capacitor C3 between pin 4 and pin 1 (T1-ACreturn) so that when subjected to large dv/dt transients the effectiveimpedance of C3 becomes negligible shunting current away from the gateof SCR SA1 to the AC return line (T1).

With reference to FIG. 2 which shows the voltage vs time curve at a ofline voltage, typically 60 Hertz frequency and at b the waveform as seenat pin 8, the resistance level of current limiting resistors R1, R2 ispreferably chosen so that the SCR conduction of the pre-regulatorsection is typically in the order of 1-2 milliseconds for a 60 Hertzfrequency occurring on the positive half of the AC cycle. Limitingconduction of the SCR of the pre-regulator section is particularlyadvantageous in an industrial environment subject to noisy lines inorder to ensure that the safe operating range of SCR SA1 will not beexceeded.

A lower voltage, i.e., 12 volts, suitable for operation of the controlcircuit, to be explained infra, is provided by suitable means such as adropping resistor R28 serially connected to a 12 volt zener diode ZD1connected between the output of IC 24 and ground. Power supply section12 also includes a snubber capacitor C4 connected between the lowvoltage output at the junction of resistor R28 and diode ZD1 and ground.Alternatively, the low voltage power supply could be provided by avoltage divider comprising two resistors, if desired.

Over-temperature protection section 14 comprises a comparator 1 havingits inverting input connected, through diode D1, resistor R29 andcapacitor C9 to the junction of a reference resistor R5 and a PTCresistor which are connected between control circuit supply voltage 28and ground. Diode D1 serves to isolate resistor R5 and the PTC from anyfeedback from the supply rail 28 through resistors R16, R17 when theoutput transistor of comparator 2 is off and the inverting input is tiedto the 12 volt rail. The non-inverting input of the comparator 1 isconnected to the junction of voltage divider resistors R6, R7 connectedbetween line 28 and ground to establish a reference voltage. Feedbackresistor R8 is connected between the output of comparator 1 and thenon-inverting input. Capacitor C9 is connected between the invertingoutput of comparator 1 and ground. The RC network of resistor R29 andcapacitor C9 snubs down noise transients on the sensor input therebypreventing nuisance tripping.

The output of comparator 1 is connected to the junction of a voltagedivider in a buffer circuit portion comprising resistors R9, R10 and thejunction in turn is connected to the inverting input of comparator 4.The non-inverting input is connected to the junction of a voltagedivider comprising resistors R11, R12. Feedback resistor R13, providinghysteresis which in turn provides a clear demarcation for switching ofcomparator 4 and avoiding contact chatter, is connected between theoutput of comparator 4 and the non-inverting input. The output ofcomparator 4 is connected to the output voltage line 26 of IC 24 throughresistors R14, R15 in output circuit 18. The emitter, collectorterminals of PNP transistor Q1 are serially connected between supplyline 26 and relay coil K1 which in turn is connected to ground. The baseof the transistor is connected to the junction of resistors R14, R15.Diode D3 is coupled across relay coil K1 to prevent back EMF, therebyprotecting transistor Q1.

As stated supra, a fixed voltage reference provided by voltage dividerR6, R7 establishes a threshold level at the non-inverting input ofcomparator 1. The variable voltage provided by the PTC sensor comprisesthe variable voltage at the inverting terminal of comparator 1 whichvaries according to the change in the resistance of the PTC dependent onchanges in temperature of the PTC. When the voltage at the invertingterminal exceeds that of the non-inverting terminal the output NPNtransistor of comparator 1 is biased on thereby sinking current toground. This results in the reference voltage at the non-inverting inputof comparator 4 being greater than the voltage at the inverting input ofcomparator 4 resulting in the output transistor of comparator 4 to bebiased off preventing current flow to ground for transistor Q1 andconcomitantly de-energizing the relay coil K1 opening contacts M1, M2and, in turn, the motor. Feedback resistor R8 provides hysteresis toprevent transistor Q1 from being biased on until the PTC element hascooled to its normal operating resistance level at which time thecircuit will reset and permit energization of coil K1, contacts M1, M2and the motor.

Time delay section 20 is an optional feature comprising comparator 2whose non-inverting input is connected through resistor R21 to a pointbetween resistor R19 and diode D4 to the junction of resistors R18 andR20. Resistor R18 is connected to the supply rail 28 while resistor R20is connected to the output of comparator 1. The inverting input ofcomparator 2 is connected to the junction of resistors R18 and R20. TheRC network comprising capacitor C6 coupled around resistor R19 providesthe selected time delay. A resistance value of 1.1 MEG. ohms forresistor R19 provides a four minute delay using 100 uf for capacitor C6.By using a resistance value of 500K ohms for R19 a two minute delay isprovided. Resistor R21 prevents discharge current from capacitor C6 fromgoing back into the non-inverting terminal. The output of comparator 2is connected to the junction of resistors R16, R17 which are connectedbetween supply line 28 and the inverting input of comparator 1. Duringsteady state operation of a motor the input voltage at the invertingterminal of comparator 2 exceeds that of the non-inverting terminal dueto the voltage drop of diode D4 with the output transistor biased on.When the module trips due to an over-temperature condition, or the like,the output transistor of comparator 1 is biased on sinking current fromresistor R20 to ground thereby lowering potential between voltagedivider R18, R20 so that voltage at non-inverting terminal of comparator2 exceeds that of the inverting terminal and the output transistor ofcomparator 2 is biased off. Since the output of comparator 2 is tied tothe inverting input of comparator 1 it keeps the inverting input ofcomparator 1 tied to the supply rail 28 until capacitor C6, dischargingthrough resistor R19, decreases to a value lower than the voltage at theinverting input of comparator 2 and the circuit is then reset. Diode D4prevents any current from capacitor C6 back feeding to the supply rail.

Low voltage cut-out circuit section 22 is another optional feature andit comprises comparator 3 whose non-inverting input terminal isconnected to the junction of resistors R25, R26 which are connectedthrough diode D6 and resistor R27 between AC line voltage and groundwith capacitor C8 coupled across resistors R25, R26. The inverting inputterminal of comparator 3 is connected between resistor R23 and zenerdiode ZD2 connected between supply line 28 and ground to provide areference voltage. The output of comparator 3 is connected to the outputof comparator 1. Feedback resistor R22 and diode D5 are connectedbetween the output of comparator 3 and the non-inverting input thereof.Resistor R22 provides hysteresis to avoid problems with the ripplevoltage associated with charging of capacitor C8. When a motor is firstenergized resistor R22 and diode D5 are in parallel with resistor R26;however, once the peak of the ripple voltage charging capacitor C8exceeds the reference voltage of zener diode ZD2 causing a change ofstate of comparator 3 resistor R22 is effectively removed with thevoltage at the non-inverting input of comparator 3 going significantlyhigher, e.g., from approximately 3.2 V to approximately 5 V. CapacitorC7 is connected between the input terminals of comparator 3 to suppressany high frequency transients at the input terminals.

During normal operation the output transistor of comparator 3 is off andthe potential of the inverting input of comparator 3 is less than thepotential at the non-inverting input. When line voltage starts todecrease thereby decreasing the voltage at the non-inverting terminalbelow the preselected level thereby biasing the output transistor ofcomparator 3 on. Current flows from supply rail 28 and since the outputtransistor of comparator 1 is off the current flows to ground throughoutput transistor of comparator 3. The inverting input of comparator 4is than tied to ground making the positive or non-inverting input higherthan the negative input turning off the output transistor of comparator4, so that the base junction of transistor Q1 is no longer tied toground.

When the time delay option is used if an over-temperature or low voltagecondition is sensed a pre-programmed two or four minute delay will beactivated preventing energization of the relay (and motor) even thoughthe PTC sensor may have cooled to the reset temperature.

Preferably the time delay function is used whenever the low voltagefunction is used to prevent undesirable oscillation should the voltagehover near the designed cut-out value. In those cases where it ispreferred not to employ these functions they can easily be shorted outusing appropriate jumpers to tie the inputs of comparators 2 and 3 toground and removing jumpers J1 and J2.

A circuit built in accordance with the invention has the followingcomponent values:

    ______________________________________                                        MOV1            270 V        R1 75 ohms 3 W - 5%                              C1              .10 uf (275 VAC)                                                                           R3 2 ohms - 5%                                   C2              470 uf       R4 19K ohms - 1%                                 C3              150 pf       R5 2.67K ohms - 1%                               C4              4.7 uf (50 V)                                                                              R6 75K ohms - 1%                                 C5              .01 uf       R7 121K ohms - 1%                                C6              100 uf (25 V)                                                                              R8 182K ohms - 1%                                C7              .01 uf       R9 5K ohms - 5%                                  C8              4.7 uf (50 V)                                                                              R10 5K ohms - 1%                                 C9              .01 uf (50 V)                                                                              R11 56K ohms - 5%                                IC 24           HV3-240E-9                                                    Harris Semiconductor Co.                                                      comparator 1                 R12 10K ohms - 5%                                comparator 2    LM2901       R13 270K ohms - 5%                               comparator 3    (Texas       R14 7K ohms - 5%                                                 Instruments)                                                  comparator 4                 R15 27K ohms - 5%                                ZD1             12 V         R16 218K ohms - 5%                               ZD2             3.9 V        R17 330K ohms - 1%                               Q1                           R18 15K ohms - 1%                                (2N4403; NPN)                                                                 D1 1N 4004                   R19 1.1M ohms - 1%                               D2 1N 4004                   R20 1.8K ohms - 1%                               D3 1N 4004                   R21 470K ohms - 5%                                                            R22 75K ohms - 1%                                D4 1N 4004                   R23 2.7K ohms - 5%                               D5 1N 4004                   R25 22K ohms - 1%                                D6 1N 4004                   R26 R Select ohms - 1%                           Relay           24 V         R27 169K - 1%                                                    Omron G2R    R28 1K - 1%                                                                   R29 150K - 1%                                                                 PTC 1.0K BA Sensor                                                            (Texas Instruments)                              ______________________________________                                    

In certain industrial applications the motor protection system issubjected to radiated and/or conductive noise transient conditions whichcan cause erratic operation of IC 24 and nuisance tripping by turningoff internal switch 24c through inhibit capacitor C3 and in turn theentire system. With reference to FIG. 6 a modified embodiment of theinvention is shown which prevents such nuisance tripping. As seen in thefigure, the cathode of a diode D7 is connected to pin 4 with its anodeconnected to pin 1 and a capacitor C10 is connected between pins 4 and8. When subjected to noise transient conditions the high impedanceimposed on pin 4 by pull-up capacitor C10 and reversed biased diode D7will prevent SCR's SA1 and SA2 from turning off thereby preventing themfrom functioning erratically during these adverse conditions andallowing them to function in a manner similar to normal mode operation.MOV1 and the front end filter network comprising resistors R31, R32 andR33 and capacitor C1 are increased in value relative to FIG. 1 toenhance filtering capability to reduce large dv/dt transients at pin 8and prevent SCR SA1 of switch 24c from being damaged.

A circuit built in accordance with FIG. 6 has the following componentvalues:

    ______________________________________                                        MOV1   275 V        R31     64.9 ohms                                                                             3 w - 1%                                  C1     .22 uf (250 V)                                                                             R32     64.9 ohms                                                                             3 w - 1%                                  C2     470 uf       R33     64.9 ohms                                                                             3 w - 1%                                  C4     4.7 uf       R28     1K ohms 1/2 w                                     C10    4700 pf (250 V)                                                                            D7      1 N 4004                                                              ZD1     12 V                                                                  ZD3     18 V                                              ______________________________________                                    

With reference to FIG. 3 the circuit components of FIG. 1 mounted on asuitable circuit board are received in the bottom housing half 30 havingupstanding wall 32 infilled with a suitable electrically insulative butthermally conductive potting material such as two part epoxy (2850FTFRwith catalyst LN2636-101) available from Emerson and Cuming Inc. Use ofthe epoxy potting provides insensitivity to vibration and facilitateslocking of the top housing half 34 to half 30. Top housing half 34 isprovided with downwardly depending skirt portions 36, 38, 40 which arereceived inboard of wall 32 prior to the curing of the epoxy and are ofa length such that the distal free end of the skirts are spaced from thecircuit board by approximately 0.001 inch to provide suitable holdingarea for the epoxy to adhere to. To further enhance the effectiveness ofthe lock the skirts are preferably provided with a plurality of landsand grooves 42, 44 as shown in FIG. 4.

Terminals T2, T1, M2, M1 and S2, S1 project upwardly from the epoxy andare received through terminal apertures in housing half 34. As best seenin FIG. 5 terminal block members T2, T1 are smaller than the remainingterminals in order to ensure that line power will not be connected tothe output or sensor terminals. This obviates potential miswiring byfield personnel or the like which otherwise could result in damaging thecontrol circuit.

Terminal X, shown in FIGS. 1, 3 and 5 is used to disable the time delayfeature connecting point X with S1 so that other features of the circuitcan be tested more expeditiously.

Use of the power supply described above enables the over-temperatureprotection system to be used with a wide range of line voltages, from 30to 264 V_(rms) without any change in the system's components or in theway that the system is connected to line voltage. Thus theover-temperature protection portion of the system can readily be usedwith 120 or 240 VAC applications. Additional advantages are obtainedsince the transformer, along with the diode rectifier network of theprior art, is replaced by the IC 24 thereby resulting in a reduction insize and weight of the system. Cost reduction is also achieved byreplacing a conventional triac and its accessories with a low costrelay. The protection system built in accordance with the invention isnot only less expensive due to the elimination of the transformer andreduction of inventory parts it is more reliable and is easilymanufactured.

The protection system is particularly suitable for use with refrigerantcompressor motors to provide protection from overheating due to lockedrotor, running overload, light loads, loss of charge and high dischargegas temperature conditions. The system can be provided with or withoutthe low voltage cut-out and time delay features.

It should be understood that though preferred embodiments of theinvention have been described by way of illustrating the invention, theinvention includes all modifications and equivalents of the disclosedembodiments falling within the scope of the appended claims.

We claim:
 1. An electronic motor protection system for de-energizing amotor upon the occurrence of selected conditions comprising a housing,the housing containing a power supply section, control circuitry fordetecting the existence of the selected conditions and an output circuitresponsive to a signal from the control circuitry for controlling thestate of energization of the motor, the power supply section includingsolid state switch means, the solid state switch means having aswitching pre-regulator connected across line voltage adapted to turn onevery cycle for a portion of only one-half cycle beginning at zerocrossing of the AC voltage wave, the solid state switch means includingan input pin connected externally to line voltage and internally to arectifying diode and a serially connected first SCR, the first SCRhaving a gate, a second SCR having main electrodes connected to the gateof the first SCR and to an AC return line, an energy storage capacitorand a charging resistor connected to the first SCR and adapted to becharged by current flow through the first SCR, the storage capacitorcoupled to a voltage regulator, the voltage regulator having an outputvoltage, means to compare the voltage of the storage capacitor with areference and to turn on the second SCR when the voltage of the storagecapacitor exceeds the reference plus the output voltage of the voltageregulator to thereby turn off the first SCR, the means to turn on thesecond SCR includes an anode gate connected to the second SCR, a diodeconnected between the anode gate and the AC return line, and a pull-upcapacitor, the diode having a cathode, the cathode connected to theanode gate and to the pull-up capacitor, the other side of the capacitorconnected to the input pin whereby a high impedance will be maintainedat the anode gate by the pull-up capacitor and the diode together andwill prevent current flow to the AC return line when subjected totransient noise conditions,a feedback component external to the solidstate switch means coupled to the voltage regulator to provide apreselected level of DC output voltage, the output circuit comprising arelay coil serially connected to a solid state switch having a base, thecoil and solid state switch being connected across the preselected levelof DC output voltage, the control circuitry coupled to the power supplysection comprising a voltage divider network including a temperaturedependent resistor for developing a voltage which varies in accordancewith the existence and non-existence of the selected conditions, firstcomparator means having a fixed reference voltage for comparing thedeveloped voltage with a reference voltage, the first comparator meanshaving an output coupled to the base of the solid state switch in theoutput circuit and being adapted to control the conduction of the solidstate switch and concomitantly the energization of the relay coil independence upon whether the developed voltage is above or below thereference voltage.
 2. An electronic motor protection system according toclaim 1 in which the power supply section includes step down means toprovide a selected lower DC voltage for the control circuitry.
 3. Anelectronic motor protection system according to claim 2 in which thestep down means includes a serially connected current limiting resistorand zener diode.
 4. An electronic motor protection system according toclaim 1 further including a buffer circuit comprising a secondcomparator means having an inverting input terminal interposed betweenthe first comparator means and the output circuit, the output of thefirst comparator means connected to the inverting input terminal and theoutput of the second comparator means connected to the base of theoutput circuit solid state switch through a resistor.
 5. An electronicmotor protection system according to claim 4 further including a timedelay circuit comprising a third comparator means having an invertingand a non-inverting input terminal, the inverting input terminal coupledto the output of the first comparator means, an RC network coupled tothe non-inverting input terminal of the third comparator means and theoutput of the third comparator means coupled to the inverting input ofthe first comparator means.
 6. An electronic motor protection systemaccording to claim 1 further including a low voltage circuit comprisinga fourth comparator means having an inverting and non-inverting inputterminals, the non-inverting input terminal coupled to the line voltageand the inverting input terminal coupled to a control circuit voltage,the output of the fourth comparator means coupled to the output of thefirst comparator means.